1. Field of the Invention
The subject invention relates generally to the field of communications and more particularly to circuitry designed to provide power and a clock signal across a high voltage isolation barrier.
2. Description of Related Art
Digital telephone coder-decoder circuits [CODEC""s] are known in the prior art for performing digitization of voice signals. In order to operate a CODEC connected directly to a telephone line, it is necessary to provide a system clock and power to the line side of the high voltage isolation barrier. For a variety of reasons, locating a CODEC on the line side of the high voltage isolation barrier appears to the inventors to be a good approach to creating a VLSI device for a low cost, high performance telephone interface.
Conventionally, power has been transferred across the high voltage isolation barrier by a DC-to-DC converter or derived from the telephone line loop current. A system clock is sent across the barrier by a separate electrical circuit, extracted from the data, or generated locally on the line side, which is both expensive and inaccurate.
According to the invention, a capacitive isolation device is constructed having a voltage rating selected for such purpose. A charge storage device is provided for accumulating charge and developing a line side voltage. The circuit is further designed such that when the capacitive isolation device is driven by a voltage signal of alternating polarity located on the system side of the barrier, the charge storage device accumulates a charge which produces the line side voltage, while at the same time a clock signal at the frequency of the signal of alternating polarity is transferred to the line side.
The invention further contemplates connecting first circuitry to the system side of a telephone line interface which facilitates transmission of a digital signal to the line side of the interface, as well as provides isolation between the system side and the line side, and then connecting said first circuitry to second circuitry which stores a voltage in response to application of the digital signal to the first circuitry.
The approach of the invention can be advantageously employed to provide a very reliable clock signal from a source on the system side of the high voltage isolation barrier to a CODEC or other component located on the line side of the high voltage isolation barrier. Jitter and duty cycle changes are minimal. When the CODEC is not operating, inactive sections of the line side VLSI device are preferably shut down by a system control circuit to conserve power.